The present invention relates to articles of softened polymeric material and/or articles of polymeric material which have portions of differing softness, pliability or flexibility, and to a method for making such articles. More particularly, this invention relates to medical catheters, such as thin-walled angiographic catheters, guiding catheters, angioplasty catheters, urinary tract catheters, gastroenterology catheters, and the like, and to a method for making such catheters wherein the catheter includes an elongated body or shaft portion and a softer or more pliable or flexible distal end portion which is integral to its adjacent shaft portion. Even more specifically, this invention relates to a three zone catheter that includes 1) a relatively stiffer body portion, 2) a softer, or more pliable or flexible distal end portion which may terminate in 3) an even softer or more pliable or flexible tip end portion.
Catheters are widely used in the medical field for a variety of applications, including both diagnostic and therapeutic procedures. Depending on the particular medical application, it is often desirable for different parts of the catheter to exhibit different physical characteristics. For example, radiological catheters are widely used in angiographic applications where it is necessary to administer a fluid at a location within the cardiovascular system of a patient. Because these catheters are inserted into and passed through the blood vessels of the vascular system, they necessarily must have a very small outside diameter. On the other hand, because the radiologist usually desires to administer large boluses of radiopaque dyes or the like at high flow rates to obtain the sharpest x-ray image, the catheter lumen should have the largest possible inside diameter (i.e., the catheter should have the thinnest possible wall). However, competing considerations which limit the thinness of the catheter wall are the need for high strength to withstand the high pressures of liquid injection, which may exceed 1000 psi, and the need for a high degree of tensile strength and stiffness in order to allow the catheter to be pushed, guided and manipulated through the vascular system of the patient without kinking or buckling. Accordingly, in addition to being thin-walled, catheters such as angiographic catheters should have a shaft or body portion with good mechanical stability, burst strength and kink resistance.
Yet, it is often desirable for the same catheter to have a more pliable distal end portion. For example, many specialty catheters such as diagnostic catheters and urinary stent catheters are shaped at a distal end portion into specific configurations to reach difficult vessels and/or to locate a portion of the catheter within the various vessels, sinuses and cavities. The distal end portion of a so-called Judkins catheter, as an example, is bent into a bell-like shape to help insert the tip into the coronary arteries. The distal end portion of a coronary diagnostic catheter is typically formed into a pig-tail or loop to allow lodgement of the catheter tip in the heart ventricles. Similarly, a urinary stent may utilize a double pig-tail, i.e., one on each end to allow lodgement of the catheter tips in the kidney calyxes as well as the urinary bladder. Because many of these catheters are inserted into the patient along and over guide wires, it is desirable for this bent or formed portion to be more flexible or pliable than the body portion of the catheter to enable sliding of the catheter over the guidewire without significantly distorting the guidewire.
Further, because catheters typically must be able to reach distant vessels within the body without damaging, tearing or causing trauma to the various tissue, the tip end portion of the catheter is preferably even softer and less traumatic than the body and adjacent portion of the distal end of the catheter. Unless the context indicates otherwise, for purposes of this description "distal end" means generally the portion or area at the end of the catheter which is located farthest from the physician or other medical personnel using the catheter. The term "mid-distal end" is used herein with reference to a part of the distal end portion which does not extend to the very tip end of the catheter. "Tip end" is used herein to refer to the most distal end portion of the catheter. For example, an angiographic catheter of the type which must be passed through blood vessels and/or into the ostium of a coronary artery should not cause trauma to the lining of the vessel or to other tissue, such as a heart leaflet valve. Accordingly, in order to reduce the risk of trauma, it is desirable that such a catheter, while having a body and/or mid-distal end portions which meet the above criteria, also have an even softer or more flexible tip or tip end portion.
The prior art has attempted in several different ways to address the need for a catheter which has different portions exhibiting different characteristics. As described, for example in U.S. Pat. No. 4,551,292, a catheter with a more rigid body and a softer tip may be provided by separately molding the body portion and the distal or tip portion of different materials and joining them by either heat sealing, sonic sealing, solvent bonding or other fusing techniques.
One drawback with such a "two-piece" or "three-piece" catheter, however, is the possibility that the distal or tip end may separate from the body of the catheter during a medical procedure. Fused multi-piece catheters have an abrupt transition from the harder portion end to the fused-on softer portion. This abrupt transition focuses stresses on the fuse joint which renders it more susceptible to kinking and further risks separation. Regardless of the medical procedure, inadvertent separation of the distal or tip end from the catheter body or shaft during the procedure is to be avoided. With vascular catheters such as angiographic catheters which are used in cardiovascular procedures and are often used in close proximity to the heart, separation can create risk of serious harm to the patient. For example, a separated catheter may reduce blood flow to the heart or actually lodge within the heart and require immediate surgery to remove. In the best of circumstances, a separated end is a great inconvenience for the surgeon and, at worst, is life-threatening and possibly fatal to the patient.
Unfortunately, the potential for end separation is increased in thin wall catheters, such as angiographic catheters or guiding catheters, where the reduced wall thickness may not provide a sufficient amount of cross-sectional area to allow for the secure attachment of the distal or tip end portion of the catheter to the catheter body or shaft. For similar reasons, thin wall catheters of two-piece or three-piece construction require more precise, laborious and time-consuming assembly techniques and test procedures to better insure the security of the bond between the end and shaft and to provide smooth, continuous and confluent inner and outer walls of the catheter to prevent snagging of the catheter within a guiding catheter or of a guidewire within a catheter lumen. Finally, fused-multi-piece construction makes assembly of multilumen catheters extremely difficult.
Accordingly, the prior art also discloses several efforts to provide a catheter having a softer end portion but wherein the end portion is of a one-piece or fuseless construction with the body portion of the catheter. For example, U.S. Pat. No. 4,753,765 describes a catheter having a two-layered tubular body portion with a rigid inner sheath and a more flexible outer sheath. The end portion is an integral extension of the flexible outer sheath that is co-extruded over the rigid inner sheath and extends beyond the distal end of the inner sheath.
The precision co-extrusion process described in U.S. Pat. No. 4,753,765, however, has many drawbacks. One drawback is that the extrusion is difficult to control, particularly for thin wall catheters, as it utilizes an underlying material having a significantly higher melting point than the top extruded material. This dissimilarity of materials may also adversely affect bonding. Another drawback is that thin tubes require a wire mandrel support for the tubing which can be quite costly. Also, the process described in U.S. Pat. No. 4,753,765 is not conducive to mass production because the underlying sheath is discontinuous, and this may also significantly increase the expense of manufacturing these catheters. Another drawback with this type of catheter is the abrupt transition between the hard and soft sheaths. Finally, the requirement for multiple layers, in particular for a three zone catheter (having a body or shaft portion of certain hardness and a softer mid-distal end portion and an even softer tip end portion), may increase the overall outside diameter of the catheter and/or reduce the diameter of the lumen, both of which are to be avoided.
The prior art also discloses a catheter body and tip of relatively soft material, with braiding provided along the body portion to provide the desired stiffness and burst resistance. Braiding a thin wall catheter, however, is a difficult and expensive process which results in greater waste, higher than desirable wall thicknesses, and increased costs. Moreover, braiding a three zone catheter is even more difficult and costly to perform.
Another prior art patent, U.S. Pat. No. 4,960,410, describes a two piece construction having a helix cut through the wall of the distal tip of the catheter to provide flexibility and pushability. The portion is then sheathed with a thin softer tube to complete the catheter wall. One drawback with this catheter and method for making such a catheter is that cutting the helix, sheathing and bonding the sheath to the inner catheter is both laborious and costly. In addition, although this construction provides a pliable or flexible distal zone, it does not provide a softer end or tip portion that is atraumatic to the blood vessel lining.
Another technique for forming a fuseless catheter of differing characteristics is described in U.S. Pat. No. 4,963,306. This patent discloses a novel method of making a fuseless, thin-walled catheter without co-extruding multiple layers of plastic, braiding or bonding a separate tip. In general, the method includes subjecting the body or shaft portion of an extruded polymeric tube of the catheter to solid state polymerization, while shielding or excluding the distal portion from the solid state polymerization process. The effect of the solid state polymerization process is to make the body or shaft portion of the catheter harder and more rigid. The distal portion, however, in which solid state polymerization has been retarded, remains softer and more pliable. Accordingly, although solid state polymerization effectively hardens a polymer and can be used to harden the body portion of a catheter relative to the distal portion, it cannot be used to soften an existing polymer or catheter tube, for example, for making the very tip end of an existing catheter tube even softer and atraumatic.
Still another prior art patent, Japanese Patent No. 1-17384, describes an integral soft-tip catheter made by dipping the leading edge of the catheter into a solution of a plasticizer dissolved in a solvent that is a good solvent for the plasticizer but a "poor" solvent for the polymer comprising the catheter. A "poor" solvent for the polymer is identified as a solvent that will not cause the polymer to swell or otherwise adversely effect the polymer comprising the catheter.
Unfortunately, the method described in Japanese Patent No. 1-17384 requires a substantial amount of time (approximately four hours for thin-walled catheters) to even minimally plasticize the intended flexible portion. Further, this method limits the plasticizer that can enter the polymer to relatively small mobile molecules of the plasticizer. When the catheter is used, these smaller molecules can also more easily migrate out of the catheter and increase the possible risk of adverse tissue reactions. The higher molecular weight, low mobility plasticizers such as hexylbenzenesulfonamide are not readily imbibed or absorbed in sufficiently substantial amounts into the polymer system with these solvents.
Finally, the process described in Japanese Patent No. 1-17384, utilizing a "poor" solvent, does not permit the desired amount of plasticizer, typically approximately 25% by weight, to be absorbed into the polymer for a particularly soft tip. Thus, while the amount of plasticizer absorbed may be sufficient to somewhat reduce the bending moment or increase the flexibility of the distal portion of a catheter, it is insufficient to provide a distal tip end of a catheter which is particularly soft so as to further minimize the risk of trauma to the lining of blood vessels and/or tissue.